1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus (to be referred to as an MRI apparatus hereinafter) and, more particularly, to improvement of a radio-frequency coil (to be referred to as an RF coil hereinafter) for receiving a magnetic resonance signal (to be referred to as an MR signal hereinafter) from an object to be imaged or examined.
2. Description of the Related Art
As an example of such an RF coil, a surface coil, mainly used for spine array imaging, for obtaining an MR signal from a surface region of the object near the coil is known. It is generally known that a signal-to-noise ratio (to be referred to as an S/N ratio hereinafter) is improved when a plurality of small surface coils are used in place of one large surface coil. An apparatus using these small surface coils is described in U.S. Pat. No. 4,825,162 (Roemer et al.), "The NMR Phased Array", MAGNETIC RESONANCE IN MEDICINE 16, 192-225 (1990), and U.S. Pat. No. 5,086,275 (Roemer). According to these prior arts, a plurality of surface coils are one-dimensionally arranged, the surface coils are simultaneously operated to receive the MR signals, images are reconstructed from the respective output signals from surface coils, and the images are synthesized to obtain one image. This surface coil array is called an MRI phased array coil because the above operation is similar to that of a phased array antenna in an antenna technique.
Another prior art of the MRI phased array coil is described in "Volume Imaging with MR Phased Arrays", MAGNETIC RESONANCE IN MEDICINE 18, 309-319 (1991). This prior art relates to a phased array for abdominal imaging and is constituted by four surface coils covering an abdomen of the object.
In order to further improve an S/N ratio, a method of two-dimensionally arranging surface coils is described in the above United States Patents.
However, when the surface coils are two-dimensionally arranged, the number of surface coils is considerably increased. For example, the number of surface coils of a two-dimensional array which can cover the same field as that of a one-dimensional array constituted by four surface coils reaches 10. Consequently, 10 receivers and 10 data acquisition systems (to be referred to as DASs hereinafter) which process MR signals are required. The receivers and DASs include high-speed and high-precision A/D converters, and the cost of the receivers and DASs is increased. Therefore, a large number of receivers and DASs are not practically arranged. In addition, when data are simultaneously acquired by 10 surface coils, the amount of data exceeds the processing capacity of a data processing system depending on a pulse sequence used in this data processing. In this case, a buffer memory having a large capacity (e.g., 100 MB or more) must be additionally arranged, thereby increasing the cost. Furthermore, since a synthesized image is formed such that a set of 10 data are subject to some processing, a time required for the forming the synthesized image is 10 times a time required for reconstructing an image from raw data received by a normal single RF coil. A decrease in time required for reconstructing an image to improve the operating efficiency of a prevailing MRI apparatus is an important target. Therefore, it is not desirable to increase the reconstruction time even though an S/N ratio is improved.
Another method of using a quadrature surface coil (to be referred to as a QD surface coil hereinafter) to further improve an S/N ratio is described in U.S. Pat. No. 4,721,913 (Hyde et al.). However, a sufficient S/N ratio for a large field cannot be obtained by merely using the QD surface coil. The QD surface coil has an S/N ratio 1.3 times higher than that obtained by the non-QD surface coil covering the same field. However, when the field is enlarged, it is easy to acquire a noise originated from the object and an S/N ratio is degraded. Even in the case of the QD surface coil, a trade-off between the width or length of the field and the S/N ratio cannot be avoided.
Another prior art is described in a copending U.S. patent application Ser. No. 07/701,025 (Mori), "Multiple Coil Type Magnetic Resonance Imaging System including Filters with Different Passbands", filed Jul. 16, 1991 and assigned to the same assignee as that of the present invention. This application relates to a different technique other than the phased array to keep an S/N ratio at a high level for a long field. Plural surface coils are one-dimensionally arranged as in the MR phased array coil. The plural coils are arranged in a frequency encoding direction. Narrow band band-pass filters are respectively connected to the outputs of the coils, the outputs of the filters are synthesized, and the synthesized signal is subject to detection, A/D conversion, and data acquisition processings. According to this scheme, there is no need to provide additional data acquisition channels and to increase data processing ability so that the similar performance as that of the MR phased array coil is obtained in the case of the spine array imaging. However, this scheme has such a restriction that the coils must be arranged in a frequency encoding direction. Therefore, this scheme cannot be applied to other imagings than the spine array imaging. There is no such a restriction in the case of the MRI phased array coil.